Add JavaDoc to 'Json' classes

[sbabcoc]

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Description

In this PR, I'm adding JavaDoc headers to the classes and methods of Selenium's Json API.

Motivation and Context

The Selenium Json API is a very useful built-in mechanism for consuming and producing JSON representations of Java objects. This facility can be very useful, especially when interacting with GraphQL web service endpoints, but the lack of documentation can result in considerable confusion.
This PR adds JavaDoc headers to classes, methods, and defined constants to assist folks in using the Selenium Json API in their own code.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[diemol]

Thank you, @sbabcoc!